1. Field of the Invention
The present invention relates to an exhaust purifying apparatus for an internal combustion engine, and especially relates to an exhaust purifying apparatus for an internal combustion engine for purifying the exhaust gas by reducing the nitrogen oxides in the exhaust gas.
2. Background Art
Conventionally, it has been known that, when the air-fuel ratio of the mixture supplied to the internal combustion engine is made leaner than the normal mixture (with a stoichiometric ratio) and so-called lean-burn is executed, the output of nitrogen oxides increases. Thus, a conventional technique has been adopted to carry out the purification of the exhaust gas by providing in the exhaust system an exhaust gas purifying apparatus, which includes NOx absorbents for removing NOx by absorbing or adsorbing NOx from the exhaust gas.
The NOx absorbent absorbs or reduces NOx depending upon the oxygen concentration of the exhaust gas. When the air-fuel ratio of the mixture is made leaner than the normal air-fuel ratio and the oxygen concentration in the exhaust gas is high, the NOx absorbent absorbs NOx. In contrast, when the air-fuel ratio is made richer than normal air fuel ratio and the oxygen concentration is relatively low and the concentrations of hydrocarbons or carbon monoxide is high, the NOx absorbent purifies the exhaust gas by reduction of NOx and exhausts nitrogen gas.
On the other hand, the fuel or the lubricant for the internal combustion engine often contains sulfur, so that sulfur compounds such as sulfur oxide are contained in the exhaust gas. The above NOx absorbent absorbs sulfur compounds in addition to NOx in the exhaust gas. The sulfur compounds absorbed in the NOx absorbent are more likely to be accumulated than NOx, so that, as accumulated sulfur compounds increases, absorption of NOx to the NOx absorbent is disrupted and the purification efficiency is reduced as the accumulated sulfur compounds increase, which causes degradation of the NOx absorbent by so-called catalyst poisoning.
When such poisoning of the NOx absorbent occurs, a conventional method is known to restore the NOx absorbent by removing the sulfur compounds absorbed in the NOx absorbent by suitably heating the NOx absorbent and maintaining the NOx absorbent for a suitable time in a condition in which the air-fuel ratio is richer than the stoichiometric air-fuel ratio. Japanese Patent Application, First Publication No. Hei 7-186785 discloses a control device for controlling the temperature of the NOx absorbent provided in an internal combustion engine. The above control device controls the temperature of the exhaust gas by controlling a gear position of the transmission connected to the internal combustion engine.
According to the above document, the NOx absorbent used in the exhaust purifying apparatus of an internal combustion engine absorbs sulfur compounds as follows when an air-fuel ratio of the mixture is made lean. That is, the sulfur compounds in the exhaust, for example, SO2, is oxidized on the platinum catalyst Pt into sulfate ions such as SO3xe2x88x92 or SO4xe2x88x92, and BaSO4 is formed by bonding with BaO contained in the NOx absorbent. When the amount of the sulfur compounds absorbed in the NOx absorbent exceeds a predetermined value, the gear position of the transmission is automatically set to a low speed position and the air-fuel ratio is made rich.
That is, change of the rotation of the internal combustion engine to a low speed raises the exhaust temperature, and the temperature of the NOx absorbent reaches around 600xc2x0 C. Thereby, BaSO4 formed in the NOx absorbent is decomposed by high temperature to generate SO3xe2x88x92 or SO4xe2x88x92. As the oxygen content in the exhaust is reduced, sulfate ions such as SO3xe2x88x92 or SO4xe2x88x92 contained in the NOx absorbent are reduced and released from the NOx absorbent due to the increased content of the unburned gas containing hydrocarbons or carbon monoxide. Accordingly, the sulfur compounds absorbed in the NOx absorbent are reduced and released.
The above conventional control device of the internal combustion engine controls the temperature of the NOx absorbent by raising the temperature of the NOx absorbent in the exhaust purifying apparatus. However, a problem arises that since the temperature of the NOx absorbent is raised by setting the gear position of the transmission to a low speed, the fuel consumption and the mode of driving operation are degraded.
It is therefore an objective of the present invention to provide an exhaust purifying apparatus capable of efficiently avoiding degrading the fuel consumption and maintaining the mode of the driving operation when removing the accumulated sulfur compounds by raising the temperature of the NOx absorbent at high temperature.
An exhaust purifying apparatus, provided in an exhaust system of an internal combustion engine and provided with a nitrogen oxides absorbent, which absorbs nitrogen oxides when the air-fuel ratio of mixture is made leaner than the stoichiometric air-fuel ratio, and reduces nitrogen oxides when the air-fuel ratio is made richer than the stoichiometric air-fuel ratio, comprising: an air fuel ratio controlling device (described later in the embodiment as a fuel injection valve 16) for controlling the air-fuel ratio of said internal combustion engine, and a fuel feeding device (described later in the embodiment as a fuel feeder 29) for feeding fuel to an upstream side of said nitrogen oxide absorbent; a temperature raising device for raising the temperature of the nitrogen oxide absorbent by feeding fuel by means of said fuel feeder, when the absorbing capability of the nitrogen oxide absorbent is catalyst poisoned by the accumulated sulfur compounds; and a restoring device (described later as ECU 15) for restoring the nitrogen oxide absorbing capability of said nitrogen oxide absorbent by controlling the air-fuel ratio so as to become richer than the stoichiometric air-fuel ratio by said air-fuel ratio controlling device.
According to the exhaust purifying apparatus of an internal combustion engine having the above described structure, at the time of raising the temperature of the nitrogen oxide absorbent, the temperature of the nitrogen oxide absorbent is raised by simply supplying fuel by the fuel supplying device disposed in upstream of the nitrogen oxide absorbent, which results in preventing the degradation of the fuel consumption, in contrast to the conventional method of heating the nitrogen oxide absorbent by increasing the rotation of the internal combustion engine.
In addition, since the present apparatus does not require any auxiliary heating devices such as heaters or burners for heating the nitrogen oxides absorbent, the present apparatus can be simple and can reduce the manufacturing cost.